You'll probably get answers all over the board, so first of all, do
what you're comfortable doing.
Normally, if you're incorporating printed material, it's easier to
get away with using writing and headlines because normally you're
going to use only a tiny part or change the feel entirely so that
it's not recognizable from a particular source. But, you can run into
trouble if you take, say, an entire passage from a popular book
without attributing it. If you attribute, get permission - in
writing. Forget using a "branded name" or title unless you're doing a
humor or parody piece.
I can say newspapers/mags aren't going to care if you cut out
headlines or letters to rearrange in your art.
Photos, textiles are another story. It's a gray area. Again, do what
you're comfortable doing. A true montage, where no one item is
recognizable from a particular source probably won't get you in
For every guideline you get quoted, everyone could come up with a
"what about .." exception. How did Andy Worhol get away with the soup
can prints using a trademarked product label and name? How can
artists use bottle caps in bracelets without paying a royalty to
Coke, for instance?
It's a big fizzy can of noodles. If your art gets hugely noticed,
some companies might choose to come after you. It can happen, but
rarely. So relax, go with what you feel is OK. I can add that if you
further manipulate the image, say scan it in and then alter it so
that it's got your vision --- the hand of the artist --- you're very
safe from a lawsuit.
Congrats on donating your work for such a good cause. You should be
applauded for that!